There is a case where directed pickup does not work, which is the following:

GivenyouhaveauserUwithalineoftype"customized"GiventhiscustomlineisusingDAHDItechnologyGiventhisuserisamemberofgroupGWhenacallismadetogroupGThenyouwon't be able to intercept the call made to U by pressing *8<line number of U>

If you find yourself in this situation, you’ll need to write a bit of dialplan.